In the medical diagnosis and treatment of various brain disorders, including epilepsy, Parkinson's disease, sleep disorders, and psychiatric ailments, it is customary and frequently useful to analyze electrical signals originating in the brain. For a review of this technology, see Ajmone-Marsan, C. Electrocorticography: Historical Comments on its Development and the Evolution of its Practical Applications, Electroencephalogr. Clin. Neurophysiol, Suppl. 1998, 48: 10–16; there are numerous applications. In common usage, the term “EEG” is often used to refer to signals representing aggregate neuronal activity potentials detectable via electrodes applied to a patient's scalp, though the term can also refer to signals obtained from deep in the patient's brain via depth electrodes and the like. Specifically, “EcoGs” refer to signals obtained from internal electrodes near the surface of the brain (generally on or under the dura mater); and EcoG is a particular type of EEG. Unless the context clearly and expressly indicated otherwise, the term “EEG” shall be used generically herein to refer to both EEG and EcoG signals, regardless of where in the patient's brain the electrodes are located.
It is also becoming accepted to apply electrical stimulation to various structures of the brain for both diagnostic and therapeutic purposes. For an exemplary diagnostic application, see Black, P. M. & Ronner S. F., Cortical Mapping for Defining the Limits of Tumor Resection, Neurosurgery 1987, 20:914–919, which addresses the use of electrical stimulation via deep brain electrodes to identify functional portions of the brain prior to and as a planning stage in surgical resection. For an example of a therapeutic application, see Cooper, I. S. & Upton, A. R. M., Effects of Cerebellar Stimulation of Epilepsy, the EEG and Cerebral Palsy in Man, Electroencephalogr. Clin. Neurophysiol. Suppl. 1978, 34:349–354. In both of these examples, acutely implanted brain electrodes are connected to external equipment.
It is also contemplated that chronic stimulation can be used as a direct treatment for disorders such as epilepsy. See, e.g., U.S. Pat. No. 6,016,449 to Fischell, et al., which describes an implantable neurostimulator that is coupled to relatively permanent deep brain electrodes.
Although it is frequently possible to employ scalp electrodes for certain types of EEG monitoring and analysis, it has been found that ambient electrical noise (such as from the 50/60 Hz power source) can adversely impact signal-to-noise ratio, and certain signal components of interest may be filtered out by the patient's intervening cranium and scalp tissue. Moreover, precise localization is less feasible with scalp electrodes.
Accordingly, intracranial signal analysis, that is, the consideration of signals that originate from a patient's cranium, whether by internal or external apparatus, is best accomplished with brain surface electrodes, such as strip and grid electrodes, cortical depth leads, or some combination of surface electrodes and depth leads.
Typical brain surface strip and grid electrodes arrays consist of flat, disc-shaped electrodes that are placed on the surface of the patient's brain. In a typical strip or grid electrode array, each electrode has an exposed diameter of approximately 3 mm (or ⅛ inch), and the electrodes are distributed along a line (for a strip electrode array) or in a rectangular grid (for a grid electrode array) at a pitch of approximately 10 mm.
Another disadvantage associated with conventional leads is that the construction of the leads is such that electrical contact between the conductor and the electrodes is often unsatisfactory as the electrical connection between the conductor and a respective electrode can fail. Further, the installation and normal positioning of the lead places stress on both the distal and proximal portions of the lead. One type of conventional implantable electrical lead consists of a 1 mm diameter silicon tube with a conductor element being disposed within the tube and extending a length thereof. The electrical lead includes electrodes (e.g., ring electrodes) at both distal and proximal ends of the tube. Each electrode is electrically connected to the conductor at a contact point to permit current to follow therebetween. During implantation and/or use, the robustness of the electrical connection between the conductor and one or more electrodes can degrade or fail due to forces (i.e., stress) being applied to the distal and/or proximal ends of the lead. In other words, the electrodes can become dislodged from the conductor with relative ease, thereby causing the electrical connection to fail and also possibly causing the electrode to completely become dislodged from the lead. This is undesirable since it may result in the electrode being left in situ. This is an unsatisfactory result as it renders the electrical lead operating at less than optimal conditions and in order to repair the electrical lead, the electrical connection must be restored by repairing the electrical lead or by replacing the electrical lead with a different one. Both of these options are not very attractive since each requires additional surgery (with the associated risks for the patient).
Accordingly, it would be desirable to have an implantable medical electrical lead that provides improved electrical connection between the conductor and the electrodes spaced along the medical electrical lead and eliminates or reduces the likelihood that an electrode can become dislodged from the lead during implantation and/or use.